Check for $(CXX).exe as well when checking for the android toolchain

This fixes building for android on windows.

Building for android on windows still requires MSYS for running make,
but running the ndk-build of the sample projects from within MSYS
doesn't seem to work, so one still needs to build it in two steps,
first make OS=android ... libopenh264.so from within MSYS, then
building the sample projects manually.

Review at https://rbcommons.com/s/OpenH264/r/747/.